Wind farms are commonly located in remote areas to take advantage of good wind conditions. Because of sometimes extreme conditions and the distance maintenance crew may have to travel, it is sometimes desirable to incorporate redundancy into the design of WTGs.
One component of a WTG that may be subject to faults or damage is a power electronic converter. In order to provide redundancy, it is known to provide multiple converters for each WTG. In normal operation each converter operates in parallel and is rated to take a share of the power generated by the WTG. If one of the converters fails, the WTG can continue to operate albeit with the power reduced to the lower combined converter capacity.
The grid code in each country is very strict about the maximum levels of current or voltage at frequencies other than the grid frequency of 50 Hz or 60 Hz. Currents or voltages at frequencies other than the grid frequency are called harmonics and are typically generated by power electronic devices, such as the power electronic converter in a WTG. As a result WTGs typically include filtering to ensure that any harmonics generated are filtered out to a level that complies with the grid code.
One of the primary harmonics generated by the power electronic converter in a WTG, is at the switching frequency. This may typically be of the order of several kHz and may require significant filtering requirements. With multiple parallel converters there is the possibility of reducing the harmonic filtering requirement at the switching frequency, by “interleaving” the converters. In interleaving the switching waveforms of each converter are displaced in phase over a switching period. In this way the switching harmonic is spread over to higher frequencies and reduced in amplitude, which may reduce the cost of the filters.
Prior art interleaving systems generally incorporate a clock synchronisation system so that the switching waveforms of each converter are correctly displaced in phase relative to each other. For example if three converters were interleaved the switching waveforms should each be spaced 120 degrees relative to each other to give maximum benefit. In order to accurately achieve 120 degree spacing, the clocks may need to be synchronised. Accurate clock synchronisation may add to the cost and complexity of the design and/or may reduce reliability. Also if one of the converters trips out, the remaining converters may need to restart and resynchronise.